Page 367 - Water and wastewater engineering

9-6 WATER AND WASTEWATER ENGINEERING

The flow balance and mass balance are

Q Q Q (9-9)
F P C
CQ C Q C Q (9-10)
FF P P C C
where the subscripts refer to feed water ( F ), permeate ( P ), and concentrate ( C ).
Rejection is defined as

C
Rej P (9-11)
1
C F
With an assumption that the rejection is close to 100 percent, these equations can be solved for
the concentrate concentration:

⎛ 1⎞
C C F ⎜ ⎟ (9-12)
C r ⎝ ⎠

9-3 PROPERTIES OF RO AND NF MEMBRANES
Membrane Material
The materials most widely used in RO and NF are cellulosic derivatives and polyamide derivatives.
Cellulosic acetate (CA), the common commercial material, is not tolerant to temperatures
above 30 C and tends to hydrolyze when the pH is less than 3 or greater than 8. It is susceptible to
biological degradation and degrades with free chlorine concentrations above 1 mg/L. Most mem-
brane manufacturers guarantee integrity of membranes if the chlorine concentration and contact
time are within specified limits.
Polyamide (PA) membranes are generally resistant to biological degradation, are stable over
a pH range of 3 to 11, and do not hydrolyze in water. Under similar pressure and temperature
conditions, PA membranes can produce higher water flux and higher salt rejection than CA mem-
branes. However, PA membranes are more susceptible to fouling and cannot tolerate free chlo-
rine at any concentration (MWH, 2005).

Membrane Configuration
The membrane units are fabricated in either a spiral-wound configuration or a hollow-fiber con-
figuration.
The spiral-wound configuration is shown in Figure 9-4 . Two sheets of flat-sheet membrane
are joined along three sides with the active membrane layer facing out. A spacer is placed be-
tween the membrane sheets to keep them from touching. The open end of the envelope formed by
the two sheets is attached to a perforated central tube that collects the permeate. The spiral-wound
elements are typically 1 m long and 0.3 m in diameter. The area for a 1 m long element would be
2
about 30 m . Individual elements have a permeate recovery of 5 to 15 percent. To achieve higher
recoveries, elements are placed in series. Typically, four to seven elements are arranged in series
in a pressure vessel (MWH, 2005).